When you upgrade Firefox from 1.5 to 2 and are a user of the Firebug 1.0 beta, you may notice that when you inspect an element you can't see the styles - i.e. you get an error message - 'Unable to show styles. You must install DOM Inspector.'. This can happen even if you have the DOM Inspector installed. You may also notice that you cannot see the styles in that either. So the problem seems to be that the DOM Inspector is not upgraded to the latest version when you upgrade (or some old files are not removed).
To get it working, simply close all instances of Firefox, navigate to the components directory in the Firefox application folder (i.e C:\Program Files\Mozilla Firefox\components and delete all files beginning with the word inspector. Once you have done this, reinstall Firefox to the same folder. When you start Firefox again, you should now be able to edit and disable styles within Firebug.
What I have noticed, is that after reinstalling, the files that were deleted are not in the components folder, which suggests that the DOM Inspector actually uses a feature built into Firefox (i.e. it just provides a GUI interface) and it may be the case that Firebug can actually work without installing the DOM Inspector. Infact I have disabled it and Firebug still works.